1. Field of the Invention
The present invention relates to an electron detecting mechanism for use in a charged particle beam system such as a scanning electron microscope. The invention also relates to a charged particle beam system equipped with the electron detecting mechanism.
2. Description of Related Art
A scanning electron microscope forms an electron probe by focusing an electron beam onto a sample to be observed and inspected. The electron probe is raster-scanned over the sample.
The sample over which the probe is raster-scanned in this way emits secondary electrons and backscattered electrons which are to be detected. A scanned image is formed by synchronizing a brightness signal indicative of detected electrons and a scan signal for the electron probe during scanning.
FIG. 1 is a schematic block diagram showing one example of electron detecting mechanism for use in a scanning electron microscope. An electron beam source 101 consisting of an electron gun emits an accelerated beam 102 toward a sample 110. The beam 102 is then once focused by condenser lenses (not shown) and passes through an objective aperture (not shown). The beam then passes through an opening 107a in a metal sheet or plate 107 used to emit secondary electrons. The optical axis of the beam 102 is indicated by numeral 109.
The electron beam 102 passed through the opening 107a in the metal plate 107 is focused onto the sample 110 by the focusing action of an objective lens 108. As a result, an electron probe focused over the sample 110 is formed. Under this condition, the probe is raster-scanned over the sample 110 scan coils (not shown).
The sample 110 over which the electron probe is scanned in this way emits electrons to be detected, including secondary electrons (SE) 103 of lower energies and backscattered electrons (BS) 104 of higher energies.
The secondary electrons 103 produced from the sample 110 directly reach the electron-sensitive portion 106a of an electron detector 106. The sensitive portion 106a produces an electric field which is so weak that the trajectory of the electron beam 102 being the primary beam is not affected.
The secondary electrons 103 from the sample 110 are pulled to the electron-sensitive portion 106a by the electric field. Consequently, the secondary electrons 103 directly reach the electron-sensitive portion 106a and are detected by the detector 106.
On the other hand, the backscattered electrons 104 produced from the sample 110 have energies equivalent to energies of the electron beam 102 based on the accelerating voltage and so the trajectory is hardly affected by the electric field. As a result, the backscattered electrons 104 travel straight and collide against the metal plate 107.
The collision of the backscattered electrons 104 emits electrons from the metal plate 107. The emitted electrons include secondary electrons 105 which are attracted toward the electron-sensitive portion 106a of the electron detector 106 by the electric field. In consequence, the secondary electrons 105 released from the metal plate 107 are detected by the detector 106.
As a result, the secondary electrons 103 produced from the sample 110 and the secondary electrons 105 responsive to the backscattered electrons 104 produced from the sample 110 are simultaneously detected by the electron detector 106. In this case, the secondary electrons 103 and 105 are concurrently detected by the electron detector 106.
Another example of the electron detecting mechanism is described, for example, in WO 1999/046798 and has two electron detectors juxtaposed along an optical axis. Secondary electrons released from the sample are detected by one of the electron detectors located on the lower side. Secondary electrons produced in response to backscattered electrons colliding against a reflective plate located on the beam side are detected by the other electron detector located on the upper side.
In the aforementioned detecting mechanisms, the range of energies of electrons made to impinge on the corresponding electron detector can be varied by applying a voltage on the metal plate or reflective plate.
Especially, the latter structure described in WO 1999/046798 has the following advantage. If a voltage is applied to the reflective plate and if secondary electrons produced from the reflective plate are detected while varying the voltage, the detection of secondary electrons by the lower electron detector located closer to the sample is not affected thereby, because secondary electrons emanating from the sample and secondary electrons emanating from the reflective plate are detected by respective electron detectors.
In the former structure shown in FIG. 1, information carried by the secondary electrons emanating from the sample and information carried by the backscattered electrons is superimposed and detected by an electron detector. Therefore, in this structure, it has been difficult to separate the information carried by the secondary electrons from the information carried by the backscattered electrons and to detect these two kinds of information.
In the latter structure, secondary electrons emanating from the sample and backscattered electrons are detected by separate detectors and so two kinds of information based on these two types of electrons detected can be detected individually. However, regarding backscattered electrons originating from the sample, it has been difficult to separate information carried by backscattered electrons in orbits close to the optical axis of the primary beam from information carried by backscattered electrons in more external orbits and to detect these two kinds of information.
It may be conceivable to add a separate electron detector located on the upper side to the latter structure. The added detector is arranged vertically along the optical axis. Hence, the number of electron detector stages increases. This would increase the space occupied by the charged particle optical system having an electron detecting mechanism.
If the number of electron detectors is increased to thereby increase the space occupied by the charged particle optical system in this way, the size of the whole equipment is increased. In addition, the charged particle optical system is more susceptible to external disturbances, which needs to be improved.